Apparatus and method for combining light from two or more fibers into a single fiber

ABSTRACT

An apparatus and method for combining light signals carried on a plurality of input fibers onto a single receiving fiber with a high degree of efficiency. The apparatus broadly comprises the receiving fiber and a plurality of input fiber-lens assemblies, with each fiber lens assembly including an input fiber; a collimating lens interposed between the input fiber and the receiving fiber and adapted to collimate the light signal; and a focusing lens interposed between the collimating lens and the receiving fiber and adapted to focus the collimated light signal onto the face of the receiving fiber. The components of each fiber-lens assembly are oriented along an optic axis that is inclined relative to the receiving fiber, with the inclination angle depending at least in part on the input fiber&#39;s numerical aperture and the focal lengths and diameters of the collimating and focusing lenses.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT PROGRAM

The present invention was developed with support from the U.S.government under Contract No. DE-AC04-01AL66850 with the U.S. Departmentof Energy. Accordingly, the U.S. government has certain rights in thepresent invention.

RELATED APPLICATIONS

The present application is a nonprovisional patent application andclaims priority benefit, with regard to all common subject matter, ofearlier-filed U.S. nonprovisional patent application titled “APPARATUSAND METHOD FOR COMBINING LIGHT FROM TWO OR MORE FIBERS INTO A SINGLEFIBER”, Ser. No. 10/777,561, filed Feb. 12, 2004. The identifiedearlier-filed application is hereby incorporated by reference into thepresent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates broadly to devices, apparatuses, systems,methods, and techniques for combining light from two or more opticalfibers onto a single optical fiber. More particularly, the presentinvention concerns an apparatus and method for combining light signalscarried on two or more input optical fibers onto a single receivingoptical fiber. A plurality of fiber-lens arrangements, each comprisingan input fiber, a collimating lens, and a focusing lens, are orientedalong optic axes that are inclined relative to the receiving fiber, withthe inclination angle depending at least in part on the input fibers'numerical aperture and the focal lengths and diameters of thecollimating and focusing lenses.

2. Description of the Prior Art

It is often desirable to combine light signals carried on two or moreinput optical fibers onto a single receiving optical fiber of the sametype and diameter. Various techniques are known in the prior art forcombining light signals, with some relying, for example, on complex lensarrangements and others relying on light deflecting/transmittingassemblies using fibers of different diameters and numerical apertures.None accomplish the desired combination efficiently, however, in partbecause the light exiting the input fibers is divergent with an includedangle of approximately 25° (corresponding to a standard fiber numericalaperture of 0.22). Thus, these prior art techniques are undesirablyinefficient, preserving only a small percentage of the light of thelight signals. Furthermore, apparatuses implementing these prior arttechniques typically occupy or otherwise require an undesirably largeamount of space, thereby prohibiting their use in space-limitedapplications.

Due to the above-identified and other problems and disadvantages in theprior art, a need exists for an improved apparatus for and method ofcombining light signals from multiple fibers onto a single receivingfiber.

SUMMARY OF THE INVENTION

The present invention overcomes the above-described and other problemsand disadvantages in the prior art with an improved apparatus and methodfor combining light signals from multiple fibers onto a single fiber.The apparatus broadly comprises a plurality of input fiber-lensassemblies and a receiving fiber. Each input fiber lens assemblyincludes an input optical fiber; a collimating lens; and a focusinglens, all of which are oriented along an optic axis which is inclined bya particular inclination angle relative to the receiving fiber. Theinput fiber carries a light signal into the apparatus, while thereceiving fiber receives and carries the light signal out of theapparatus. The collimating lens is interposed between the input fiberand the receiving fiber along the optic axis, and acts to collimate thelight signal. The focusing lens is interposed between the input fiberand the receiving fiber along the optic axis following the collimatinglens, and acts to focus the collimated light signal onto the face of thereceiving fiber.

It will be appreciated that the apparatus and method of the presentinvention provide a number of substantial advantages over the prior art,including, for example, allowing for combining the multiple lightsignals onto the single receiving fiber with a higher degree ofefficiency and using less space than is possible in the prior art, andusing fibers of the same diameter and numerical aperture.

These and other important features of the present invention are morefully described in the section titled DETAILED DESCRIPTION OF APREFERRED EMBODIMENT, below.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described in detailbelow with reference to the attached drawing figures, wherein:

FIG. 1 is a fragmentary schematic plan view of a preferred embodiment ofthe apparatus of the present invention, wherein light signals from twoinput fibers can be combined onto a single receiving fiber;

FIG. 2 is a schematic end view of the apparatus shown in FIG. 1;

FIG. 3 is a schematic end view of the apparatus wherein light signalsfrom three input fibers can be combined onto the single receiving fiber;

FIG. 4 is a schematic end view of the apparatus wherein light signalsfrom seven input fibers can be combined onto the single receiving fiber;and

FIG. 5 is a flow diagram setting forth a series or preferred stepsinvolved in the operation of the apparatus of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the figures, an apparatus 10 and method is hereindescribed, shown, and otherwise disclosed in accordance with a preferredembodiment of the present invention. Broadly, the apparatus 10 isadapted and operable to combine light signals from multiple fibers ontoa single fiber with a higher degree of efficiency and using less spacethan is possible with prior art techniques.

Referring particularly to FIGS. 1 and 2, the apparatus 10 broadlycomprises a plurality of input fiber-lens assemblies 12 and a receivingfiber 14. Each input fiber-lens assembly 12 includes an input opticalfiber 16; a collimating lens 18; and a focusing lens 20, all of whichare oriented along an optic axis, OA, which is inclined by a particularinclination angle, θ, relative to the receiving fiber 14. The input andreceiving fibers 16,14 may be conventional optical fibers having thesame or approximately the same diameters and numerical apertures, NA,(typically, NA=0.22).

The input fiber 16 carries a light signal into the apparatus 10, whilethe receiving fiber 14 receives and carries the light signal out of theapparatus 10. The collimating lens 18 is interposed between the inputfiber 16 and the receiving fiber 14 along the optic axis OA, and acts tocollimate the light signal. The collimating lens 18 may be a convergentlenses or, alternatively, a gradient index (GRIN) lens. The focallength, f₁, of the collimating lens 18 should be large in relation tothe core diameter of the input fiber 16 so that the core diametereffectively becomes a point source. Furthermore, the diameter, d₁, ofthe collimating lens 18 should be large enough to collect the diverginglight signal from the input fiber 16. Thus:arctan(d ₁/2f ₁)≧arcsin(NA)

The focusing lens 20 is interposed between the input fiber 16 and thereceiving fiber 14 along the optic axis OA following the collimatinglens 18, and acts to focus the collimated light signal onto the face ofthe receiving fiber 14. The converging angle, α, of the focused lightsignal is a function of the diameter, d_(signal), of the collimatedlight signal and the focal length, f₂, of the focusing lens 20:α=arctan(d _(signal)/2f ₂)The diameter, d₂, of the focusing lens 20 should be slightly larger thand_(signal), but this means that the smallest angle for θ is:θ=arctan(d ₂/2f ₂)

In order to achieve maximum coupling of the light signal from thefocusing lens 20 onto the receiving fiber 14, all of the light rayswithin the focused light signal must be incident on the face of thereceiving fiber 14 at an angle not exceeding the NA of the receivingfiber 14. That is, the angles α and θ must obey the relation:α+θ≦arcsin(NA)

The collimating and focusing lenses 18,20 should be chosen so thatneither diffraction nor aberration is a limiting factor in the focusedspot size of the light signal on the face of the receiving fiber 14.With regard to diffraction, d_(signal) should be sufficiently largerelative to f₂ so that the associated diffraction-limited spot size ofthe focused light signal is less than the core diameter of the receivingfiber 14. However, d_(signal) relative to f₂ must be small enough sothat spherical aberration associated with the focused spot size from thefocusing lens 20 is substantially less than the core diameter of thereceiving fiber 14.

It will be appreciated that the apparatus 10 of the present inventionmay be adapted to accommodate substantially any number of the inputfiber-lens assemblies 12. By way of illustration and not limitation, twosuch assemblies 12 are shown in FIGS. 1 and 2; three assemblies 12 areshown in FIG. 3; and seven assemblies 12 are shown in FIG. 4. The sevenassemblies 12 of FIG. 4 are an example of higher order close-packedhexagonal symmetry, wherein the optic axis of a central one of theassemblies 12 is coaxial with the receiving fiber 14 rather thaninclined by θ. For all other cases, the angular requirements expressedabove must be met.

In exemplary use and operation, referring particularly to FIG. 5, theapparatus 10 operates as follows. As mentioned, each fiber-lens assembly12 is oriented along the optic axis OA which is inclined by θ relativeto the receiving fiber 14, as depicted in box 30. For each fiber-lensassembly 12, as the light signal exits its input fiber 16 it diverges bya particular angle (typically, 25° for NA=0.22). The collimating lens 18collimates the diverging light signal, as depicted in box 32, and thefocusing lens 20 focuses the collimated light signal onto the face ofthe receiving fiber 14, as depicted in box 34. Thereafter the lightsignals exit the apparatus 10 via the receiving fiber 14.

It will be appreciated that the apparatus and method of the presentinvention has a variety of potential applications, including, forexample, enhancing signal-to-noise ratios in fiber-based sensingsystems; facilitating the design and implementation of simultaneousmulti-component fiber sensing systems for chemical, physical, andbiological variables; and for facilitating wavelength-divisionmultiplexing (WDM) wherein each of the input fibers carries opticalenergy of a particular wavelength of interest.

From the preceding description it will be appreciated that the apparatusand method of the present invention provide a number of substantialadvantages over the prior art, including, for example, allowing forcombining the multiple light signals onto the single receiving fiberwith a higher degree of efficiency using fibers all of the same diameterand numerical aperture and using less space than is possible in theprior art.

Although the invention has been described with reference to thepreferred embodiments illustrated in the drawings, it is noted thatequivalents may be employed and substitutions made herein withoutdeparting from the scope of the invention as recited in the claims. Asmentioned, for example, the apparatus may be adapted to accommodatesubstantially any desired number of fiber-lens arrangements.

1. An apparatus for combining multiple light signals, the apparatuscomprising: a receiving optical fiber; an input optical fiber adapted tocarry a light signal, wherein the input optical fiber is oriented alongan optic axis having an inclination angle relative to the receivingoptical fiber; a collimating lens interposed along the optic axisbetween the input optical fiber and the receiving fiber and adapted tocollimate the light signal; and a focusing lens interposed along theoptic axis between the collimating lens and the receiving fiber andadapted to focus the collimated light signal onto the receiving fiber,wherein the collimated light signal has a signal diameter, the focusinglens has a focal length, and the focused light signal has a convergenceangle, and wherein the convergence angle is equal to arctan(the signaldiameter/(2 * the focal length)).
 2. The apparatus as set forth in claim1, wherein the collimating lens is a convergent lens.
 3. The apparatusas set forth in claim 1, wherein the collimating lens is a gradientindex lens.
 4. A method of combining multiple light signals, the methodcomprising the steps of: (a) orienting an input optical fiber along anoptic axis having an inclination angle relative to a receiving opticalfiber, wherein the input optical fiber carries a light signal; (b)collimating the light signal using a collimating lens, wherein thecollimated light signal has a signal diameter; and (c) focusing thecollimated light signal onto the receiving optical fiber using afocusing lens, wherein the focusing lens has a focal length and thefocused light signal has a convergence angle, and wherein theconvergence angle is equal to arctan(the signal diameter/(2 * the focallength)).
 5. The method as set forth in claim 4, wherein the collimatinglens is a convergent lens.
 6. The method as set forth in claim 4,wherein the collimating lens is a gradient index lens.
 7. A method offacilitating combining multiple light signals, the method comprising thesteps of: (a) providing a receiving optical fiber; (b) providing aninput optical fiber adapted to carry a light signal; (c) orienting theinput optical fiber along an optic axis having an inclination anglerelative to the receiving optical fiber; (d) interposing a collimatinglens along the optic axis between the input optical fiber and thereceiving optical fiber for collimating the light signal, wherein thecollimated light signal has a signal diameter; and (e) interposing afocusing lens along the optic axis between the input optical fiber andthe receiving fiber after the collimating lens, for focusing the lightsignal into the receiving fiber, wherein the focusing lens has a focallength and the focused light signal has a convergence angle, and whereinthe convergence angle is equal to arctan(the signal diameter/(2 * thefocal length)).
 8. The method as set forth in claim 7, wherein thecollimating lens is a convergent lens.
 9. The method as set forth inclaim 7, wherein the collimating lens is a gradient index lens.